1. Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo Da Vinci 32, 20133 Milano, Italy
2. Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, S3 7RH Sheffield, U.K.
3. Chair in Hybrid Nanosystems, Nanoinstitute Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, 80539 Munich, Germany
4. Istituto di Fotonica e Nanotecnologie-Consiglio Nazionale delle Ricerche, Piazza Leonardo Da Vinci 32, 20133 Milano, Italy
5. School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
6. Department of Physics, Imperial College London, London, SW7 2AZ, U.K.